CMOS or CCD image sensors are of interest in a wide variety of sensing and imaging applications in a wide range of fields including consumer, commercial, industrial, and space electronics. CCDs are employed either in front or back illuminated configurations. Front illuminated CCD imagers are more cost effective to manufacture than back illuminated CCD imagers such that front illuminated devices dominate the consumer imaging market. Front-illuminated imagers, however, have significant performance limitations such as low fill factor/low sensitivity. The problem of low fill factor/low sensitivity is typically due to shadowing caused by the presence of opaque metal bus lines, and absorption by an array circuitry structure formed on the front surface in the pixel region of a front-illuminated imager. Thus, the active region of a pixel is typically very small (low fill factor) in large format (high-resolution) front-illuminated imagers.
An effect called blooming can occur with CCD imagers. CCD imagers are most often constructed of a p or n type semiconductor substrate with a plurality of overlying pixel structures, wherein each pixel is comprised of a channel of a second conductivity type different from the substrate, and one or more gates overlying the channel. Light incident on the front surface of front-illuminated CCD devices, or on the back surface of back-illuminated CCD devices, cause charge carriers to form in the substrate. These charge carriers migrate to the channel regions of the pixel structures, where they collect in response to potentials applied to one or more gates associated with the pixel structure. As charge accumulates, the channel well under a collecting gate begins to “fill” or approach a saturation state. The charge stored in the channel is sometimes said to be stored in the “charge well” located underneath the gate overlying the semiconductor substrate. Bright sources of light can cause so many carriers to be generated in the channel region that the charge-holding capacity of a pixel can be exceeded. As a light-gathering capacity of a pixel channel is exceeded, the excess charge carriers spill over into adjacent pixels. This spillover, called “blooming.”
Referring now to FIGS. 1A-1C, a technique in the prior art for reducing or eliminating blooming in a conventional back illuminated CCD imagers is depicted. FIG. 1A is a top-down view of the back-illuminated imager with anti-blooming drain structures. FIG. 1B shows a cross-section of the back-illuminated CCD imager which depicts the structure of a single pixel 2. FIG. 1C shows a simulated channel potential profile (gate biased) for the pixel 2. Referring now to FIG. 1A and 1B, the CCD pixel 2 includes a portion of a silicon substrate 4 of a first conductivity type, with an overlying channel region 6 of a second conductivity type different from the first conductivity type. One or more gates 13 can overlay the channel region 6 separated by anti-blooming structures 8 on each side of the channel region 6. Each of the anti-blooming blooming structures 8 comprises a barrier region 10 of the first conductivity type surrounding a drain region 12 of the second conductivity type. Gates 13 run horizontally overlying a channel oxide layer 14 for isolating the conductive gates 13 from the underlying channel region 6.
Charges generated in a backside 15 of the substrate 4 are confined in the channel 6 by an application of appropriate potentials to the gates 13. Initially, when there is no accumulated charge in the channel region 6, the channel potential 16 in FIG. 1C will be high. Barrier region potentials 18 will be low, and drain region potentials 20 will be high. As charge accumulates in the channel region 6, the channel potential 16 decreases. At some point, the channel potential 16 will be level with the barrier region potentials 18, such that charges spill into the drain regions 12. The drain regions 12 are electrically connected together (not shown) and the resulting drain current, is removed from the CCD imager.
As can be seen in FIGS. 1A and 1B, the anti-blooming structure of the prior art can be implemented in a back-illuminated imager at the expense of useful pixel width. Such lateral anti-blooming structures take real-estate away from the top-portions of CCD imagers which could be used for additional pixel structures. But, as the demand for pixel density increases, there is greater need in the industry for reduced pixel width, and therefore, incorporation of lateral anti-blooming structures in back-illuminated imagers becomes physically prohibitive.
A cross-section of a front-illuminated imager in the prior art with anti-blooming structures moved away from the imaging-component upper-portions of a silicon substrate is shown in FIG. 2. The front-illuminated CCD pixel 22 includes a portion of a silicon substrate 24 of a first conductivity type, with a drain region 26 of a second conductivity type different from the first conductivity type overlaying a top portion of the substrate 24. The drain region 26 is created by an ion implantation. The ion implantation is performed at a high energy (in the order of MeV), so that the drain region 26 thus created is deeply buried in the bulk of the substrate 24. A barrier region 28 of the first conductivity type substantially overlays the buried drain 26 and is also created by means of ion implantation. Here, the energy of the ion implantation is higher than but not greater than the energy used for the ion implantation step for creating the drain region 26. As a result, the barrier region 28 overlays the drain region 26. A channel region 30 of the second conductivity type, also created by ion implantation, substantially overlays the barrier region 28. One or more gates (not shown) can overlay the channel region 30. A pixel isolation region 32, also created by ion implantation and a drain contact 34 separate channel and barrier regions of adjoining pixels and also serves to separate the channel region 30 from the drain region 26. The drain contact 36 comes in direct contact with at least a portion of the drain region 26.
Light impinging on the front side 38 of the pixel 22 creates charge carriers which migrate to the channel region 30. The charge carriers are located in the channel region 30 because the pixel isolation region 32 provides an electrical barrier to the charge carriers stored in the channel region 30. In a similar fashion, the barrier region 28 initially provides an electrical barrier to the charge carriers confined in the channel region 30. However, as charge carriers accumulate in the channel region 30, channel potential collapses, and an instant in time is reached when the channel potential is level with the barrier potential. From that point onwards, any further accumulated carriers will overcome the barrier potential, and move to the drain region 26. The excess charge carriers in the drain region 26 are collected as a drain current through the drain contact 36.
Unfortunately, this structure is of no use for a back-illuminated CCD imager. Using the front-illuminated imager structure of FIG. 2 as a guide, in a back illuminated CCD imager, light incident on the back side 40 of the imager generates carriers in the silicon substrate 24 that are siphoned away by a buried drain implant 26 before these carriers could reach a front side channel region 30.
Accordingly, what would be desirable, but has not yet been provided, is an anti-blooming structure for use in back illuminated imager array which does not share valuable front-side real estate with carrier-collecting drain regions.